1. Field of the Invention
This invention relates to a method and device for mixing under vacuum a mixture which can be obtained from a powder and a liquid and which is intended to be subsequently admitted into containers in order to produce molds to be used in dental and goldsmith fields or the like.
2. Description of the Prior Art
It is well known, in the above mentioned fields, to prepare under vacuum a mixture of the above type, by means of a so called "spatulation" or mixing operation made by hand or mechanical means within a vessel placed under vacuum and into which the liquid and powder of the mixture or "coating mass" have been previously admitted. Usually, this mixing step is continued until a coating mass homogenization is reached, which, if necessary, is promoted by vibratory means, whereupon this coating mass is admitted, usually poured, into one or more containers, so called "cylinders" located on a rotary plate below the mixing vessel and within a bell also maintained under vacuum, everything in accordance with the well known technique of mold production with very high precision, as in the dental and goldsmith fields.
Mixing and pouring under vacuum are recommanded, if not required, by the need of removing from the coating mass any gas inclusions which could negatively affect the working precision, particularly any Co.sub.2 or O.sub.2 inclusions which could exert an undesired chemical effect on the coating mass components during the well known subsequent working operations.
It has been found, however, that the technique used till now does not ensure the removal of the above-mentioned gas inclusions and, on the contrary, that during mixing under vacuum there is a tendency to obtain a porous mixture (referred to in this field as "beer effect") produced by expansion caused by the vacuum of the air included within the powder and carbon dioxide dissolved in the water. These gas inclusions in the mass transferred to the cylinders will be more and more difficult to remove, taking into account that the mass consistency continues to increase under the effect of the beginning setting.